Manufacturing method of semiconductor light-emitting element, and semiconductor light-emitting element

ABSTRACT

There are provided a setting process configured to set in a chamber an aluminum nitride substrate in which a semiconductor layer is formed on a first principal plane, and an oxide film forming process configured to heat an inside of the chamber with a water molecule being introduced in the chamber and to form an oxide film including an amorphous oxide film and/or a crystalline oxide film on a second principal plane located on an opposite side to the first principal plane of the aluminum nitride substrate.

TECHNICAL FIELD

The present invention relates to a manufacturing method of asemiconductor light-emitting element, and the semiconductorlight-emitting element, and in particular, relates to a manufacturingmethod of a semiconductor light-emitting element in which asemiconductor layer is formed on an aluminum nitride substrate, and thesemiconductor light-emitting element.

BACKGROUND ART

A semiconductor light-emitting element, for example, a light-emittingdiode (LED) of a nitride semiconductor generally includes asemiconductor stacked part in which an n-type semiconductor layer, alight-emitting layer, an electron blocking layer, and a p-typesemiconductor layer are sequentially stacked on a substrate, and anelectrode for applying a voltage to the light-emitting layer.

Then, the light generated at the light-emitting layer is emitted to theoutside of the semiconductor light-emitting element from an externallyexposed face (top face, side face) or an exposed face (back face, sideface) of the substrate. On this occasion, at a semiconductor interfaceor at an interface between the semiconductor light-emitting element andthe air, the incident light at an angle equal to or larger than acritical angle propagates in the semiconductor layer while repeating thetotal reflection from the limitation of the total reflection to bedecided by a refractive index of the interface. In the meantime, thelight is partially absorbed in the semiconductor layer itself or isabsorbed into the electrode and converted into heat, resulting in thatthe light extraction efficiency to the outside degrades and thelight-emitting strength decreases. Therefore, various concepts have beenmade to improve the light extraction efficiency.

Especially, a technology of improving the light extraction efficiency byprocessing the semiconductor element surface so that the light entersthe interface at a critical angle or smaller is often used. PatentLiterature 1 discloses a technique of forming an uneven structure inwhich a height is equal to or more than 100 nm on the surface of asemiconductor layer and/or on the surface of a sapphire substrate andthe base has cone-shaped projections with different sizes of 1 nm to 500nm, by using an organic substance that undergoes phase separation as amask for dry etching the surface of the semiconductor layer that formsan interface with its outside and/or the surface of the sapphiresubstrate. In addition, Patent Literature 2 discloses a method ofimproving the light extraction efficiency of the light-emitting element,by forming substantially polygon-shaped unevenness on the surface of theside where the semiconductor layer of the sapphire substrate is formedby using the mask and etching.

CITATION LIST Patent Literature PLT 1: JP 2003-218383 A PLT 2: JP2012-238895 A SUMMARY Technical Problem

The technologies disclosed in Patent Literatures 1 and 2, however, stillhave room for improvement in the following points.

Like the above-described technologies, in the method of forming the maskon the processed surface by use of the photolithography process or thephase separation of the organic substance and carrying out the etchingprocess with the mask, a desired uneven pattern can be formed on thesubstrate surface. However, in the method of forming the uneven patternby using the mask, several stages are necessary for the mask formingprocess. Hence, the mass productivity is bad and the manufacturing costsincrease.

Besides, in the technology of forming the uneven structure in the dryetching like Patent Literature 1, not only the processed surface butalso the internal semiconductor layer are subject to etching damages.Hence, the light output of the semiconductor light-emitting elementmight be degraded. Moreover, in the technology of providing the opticalpattern of uneven structure having the light extraction effect on thesurface (an interface between the substrate and the semiconductor layer)of the side on which the semiconductor layer of the substrate is made togrow as disclosed in Patent Literature 2, the semiconductor layer has tobe formed on the substrate surface having such an uneven structure.Hence, the semiconductor layer crystal properties might deteriorate andthe light output might be lowered.

Therefore, the present invention has been made in view of the abovecircumstances, and has an object to provide a manufacturing method of asemiconductor light-emitting element, in which the light extractionefficiency of the semiconductor light-emitting element can be improved,and the mass productivity is good, while an etching damage onto thesemiconductor layer or a deterioration in crystal property of thesemiconductor layer in the semiconductor light-emitting element is beingsuppressed, and the semiconductor light-emitting element.

Solution to Problem

As a result of earnestly studying how to address the above-describeddrawbacks, the inventors of the present invention have found out thatthe above-described drawbacks can be addressed by a manufacturing methodof a semiconductor light-emitting element, and the semiconductorlight-emitting element to be described below.

That is to say, in one embodiment of the present invention, there isprovided a manufacturing method of a semiconductor light-emittingelement, the manufacturing method including: setting in a chamber analuminum nitride substrate in which a semiconductor layer is formed on afirst principal plane; and heating an inside of the chamber with a watermolecule being introduced in the chamber to form an oxide film includingan amorphous oxide film and/or a crystalline oxide film on a secondprincipal plane located on an opposite side to the first principal planeof the aluminum nitride substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrative of a configuration example of anoxide film forming device 50 suitable for use in one embodiment of thepresent invention;

FIG. 2A to FIG. 2D are cross-sectional views illustrative of sequentialprocesses in a manufacturing method of a semiconductor light-emittingelement 100 in one embodiment of the present invention;

FIG. 3 is a SEM image in which a surface of a second principal planeprocessed in Example 1 is observed;

FIG. 4 is a SEM image in which the second principal plane processed inExample 1 is observed;

FIG. 5 is a STEM image in which the second principal plane processed inExample 1 is observed;

FIG. 6 is a graph illustrative of outputs that change over time of thesemiconductor light-emitting element subject to oxidization and crystalgrowth in Example 1;

FIG. 7 is a SEM image in which the second principal plane processed inExample 2 is observed;

FIG. 8 is a STEM image in which the second principal plane processed inExample 2 is observed;

FIG. 9 is a SEM image in which the second principal plane processed inExample 3 is observed;

FIG. 10 is a SEM image in which the second principal plane processed inExample 4 is observed;

FIG. 11 is a STEM image in which the second principal plane processed inExample 4 is observed; and

FIG. 12A and FIG. 12B are schematic cross-sectional views illustrativeof a measuring method of a thickness of an oxide film.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments (hereinafter, also referred to as presentembodiment) to carry out the present invention will be described indetail.

(Manufacturing Method of Semiconductor Light-Emitting Element)

The manufacturing method of the semiconductor light-emitting element inthe present embodiment includes a setting process of setting up in achamber an aluminum nitride (AlN) substrate in which a semiconductorlayer is formed on a first principal plane, and an oxide film formingprocess of heating the inside of the chamber with water (H₂O) moleculesbeing introduced in the chamber, and forming an oxide film including anamorphous oxide film and/or a crystalline oxide film, on a secondprincipal plane of the aluminum nitride substrate.

(Setting Process)

The setting process in the manufacturing method of the semiconductorlight-emitting element in the present embodiment is a process of settingup in a chamber the aluminum nitride substrate in which thesemiconductor layer is formed on the first principal plane.

The chamber is not particularly limited, as far as the aluminum nitridesubstrate can be set up in its inside and water molecules can beintroduced into its inner space. In the oxide film forming process, whenthe relative humidity, the temperature, and the relative pressure arecontrolled to fall within desired ranges, respectively, the chamber hasa mechanism of controlling the relative humidity, the temperature, andthe relative pressure, while monitoring them, in one embodiment of thepresent invention.

(Oxide Film Forming Process)

In the oxide film forming process in the manufacturing method of thesemiconductor light-emitting element in the present embodiment, theinside of the chamber is heated with the water molecules beingintroduced in the chamber, so as to form an oxide film including anamorphous oxide film and/or a crystalline oxide film on the secondprincipal plane of the aluminum nitride substrate.

In addition, the oxide film is formed by controlling process conditions(such as the relative humidity, the temperature, the relative pressure,and a process time), in one embodiment. The oxide film may be amonolayer of the amorphous oxide film or the crystalline oxide film, ormay be a stack of the amorphous oxide film and the crystalline oxidefilm.

Note that it is known that a natural oxide film is formed on the surfaceof the aluminum nitride substrate by exposing the aluminum nitridesubstrate in the atmosphere. However, it has been confirmed fromExamples to be described later that no improvement in the lightextraction efficiency is made in the natural oxide film, but effects ofthe improvement in the light extraction efficiency from the secondprincipal plane are brought out by the oxide film including theamorphous oxide film and/or the crystalline oxide film on the secondprincipal plane of the aluminum nitride substrate, the oxide film beingobtained by intentionally heating the inside of the chamber with thewater molecules being introduced in the chamber.

From a viewpoint of forming an oxide film that further improves thelight extraction efficiency, the relative humidity in the chamber in theoxide film forming process may be equal to or higher than 50% and equalto or lower than 100%, in one embodiment, or may be equal to or higherthan 65% and equal to or lower than 100%, in another embodiment.

In addition, from a viewpoint of forming the oxide film that furtherimproves the light extraction efficiency, the temperature in the chamberin the oxide film forming process may be equal to or higher than 100° C.and equal to or lower than 140° C., in one embodiment, or may be equalto or higher than 105° C. and equal to or lower than 121° C., in anotherembodiment.

Further, from a viewpoint of forming the oxide film that furtherimproves the light extraction efficiency, the relative pressure force(gauge pressure) in the chamber in the oxide film forming process may beequal to or higher than 0.01 MPa and equal to or lower than 0.3 MPa, inone embodiment, or may be equal to or higher than 0.01 MPa and equal toor lower than 0.1 MPa, in another embodiment.

Hereinafter, a mechanism of forming the oxide film in the oxide filmforming process will be described. In the oxide film forming process inthe present embodiment, the inside of the chamber is heated with thewater molecules being existent in the chamber to form the oxide filmincluding the amorphous oxide film and/or the crystalline oxide film onthe second principal plane of the aluminum nitride substrate. Therefore,as compared to the condition where a natural oxide film is formed, it issupposed that steam easily reacts with the second principal plane of thealuminum nitride substrate and the oxide film including the amorphousoxide film and/or the crystalline oxide film with effects of the lightextraction improvement is formed. The amorphous oxide film with effectsof the light extraction improvement remarkably is formed, in oneembodiment, when the second principal plane of the aluminum nitridesubstrate is processed at the temperature equal to or higher than 100°C., the relative humidity equal to or higher than 50%, the pressureforce higher than the atmospheric pressure.

In addition, when the temperature in the chamber is high (for example,higher than 105° C.), it has been confirmed in Examples to be describedlater that the crystalline oxide film having a surface of an unevenstructure tends to be formed. It is supposed that the crystalline oxidefilm is formed by hydrothermal synthesis.

As the oxide film including aluminum (Al), aluminum oxide, hydrationaluminum oxide, aluminum hydroxide, or a membrane in which the abovealuminum and aluminum nitride are mixed can be given, but the oxide filmincluding aluminum is not limited to them. In the oxide film formingprocess, by controlling at least one of the temperature, the relativehumidity, the relative pressure, and any combination thereof, thereaction of the second principal plane of the aluminum nitride substrateand the steam is controlled, so that the oxide film or the uneven formof the surface can be controlled.

In the oxide film forming process in the present embodiment, plasma orthe like used by dry etching is not used. Accordingly, since no etchingdamage is made onto the semiconductor layer, a reduction in thelight-emitting efficiency caused by the etching damage is suppressed.

In addition, in the oxide film forming process in the presentembodiment, after the semiconductor layer is formed, the oxide filmhaving an uneven structure on the second principal plane of the aluminumnitride substrate is formed. Such an uneven structure is formed on thesecond principal plane, instead that an optical pattern with the lightextraction effect is formed on a surface of a side where thesemiconductor layer of the substrate (an interface between the substrateand the semiconductor layer) is made to grow. Thus, since no effect isgiven to the crystal defect occurrence at the time of the semiconductorlayer growth, there is no reduction in the light-emitting efficiencycaused by the crystal defect.

(Aluminum Nitride Substrate)

As to the aluminum nitride substrate used in the manufacturing method ofthe semiconductor light-emitting element in the present embodiment, anyaluminum nitride substrate may be used as far as it includes aconstituent element mainly including aluminum (Al) and nitrogen (N).

In addition, any aluminum nitride substrate is used as far as it has thefirst principal plane on which a semiconductor layer is formed and thesecond principal plane on which an oxide film is formed in the oxidefilm forming process. The shape of the aluminum nitride substrate is notlimited in particular, may be a form of wafer, or may be a form ofindividual tip. The first principal plane and the second principal planeface each other in substantially parallel to each other, in oneembodiment.

In addition, the above aluminum nitride substrate may include variouskinds of dopants or impurities, if necessary. The above aluminum nitridesubstrate may be a polycrystal or single crystal. The single crystal isused from a viewpoint of forming the semiconductor layer with goodcrystal properties, in one embodiment.

Further, the method of manufacturing the aluminum nitride substrate isnot limited in particular. For example, aluminum nitride obtained by asublimation method, HVPE (Hydride Vapor Phase Epitaxy), MOCVD (MetalOrganic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy), or thelike can be used.

From a viewpoint of flatness and crystal property improvement of thesemiconductor layer, the first principal plane of the aluminum nitridesubstrate may be an Al plane, in one embodiment, or may be a C plane andAl plane in a hexagonal crystal, in another embodiment. Furthermore,from a viewpoint of efficiently forming the oxide film in the presentembodiment, the second principal plane of the aluminum nitride substratemay be a C plane and N plane in the hexagonal crystal, in yet anotherembodiment.

Moreover, in the manufacturing method of the semiconductorlight-emitting element in the present embodiment, from a viewpoint offurther improving the effects in the light extraction efficiencyimprovement, the second principal plane of the aluminum nitridesubstrate is a plane that has not been subject to CMP (ChemicalMechanical Polishing), in further another embodiment.

(Semiconductor Layer)

The semiconductor layer formed on the first principal plane of analuminum nitride substrate used in the manufacturing method of thesemiconductor light-emitting device in the present embodiment is notlimited in particular, as far as it emits light when electricity issupplied to the semiconductor layer.

The semiconductor layer may be a single layer, or may have a layeredstructure in which plural semiconductor layers having differentconstituent elements or different ratios of the constitution elementsare stacked. From a viewpoint of improving the light-emittingefficiency, the semiconductor layer may have a layered structure in oneembodiment. Such a layered structure may have an n-type semiconductorlayer, a light-emitting layer, an electron blocking layer, and a p-typesemiconductor layer, in another embodiment. Such a layered structurefurther includes a contact layer that lowers a contact resistance withan electrode in a region in contact with the electrode for supplying theelectricity, in yet another embodiment.

From a viewpoint of improving the light-emitting efficiency, thelight-emitting layer has a multiplex quantum well structure (MQW,Multi-Quantum Well), in one embodiment.

From a viewpoint of controlling an emission wavelength, thesemiconductor layer may be a compound semiconductor, in one embodiment,may be a group III-V compound semiconductor, in another embodiment, maybe a group III-V compound semiconductor layer including an elementselected from a group consisting of aluminum, gallium, nitrogen andindium, in yet another embodiment, or may be a nitride compoundsemiconductor, in further another embodiment. A constitution element ora composition ratio of the semiconductor layer can be selected invarious ways depending on what light of wavelength should be emitted.When the nitride compound semiconductor is used, for example, galliumnitride, aluminum nitride, indium nitride, boron nitride, or a mixedcrystal thereof can be used, but the nitride compound semiconductor isnot limited to them.

In addition, from a viewpoint of extracting the light effectively fromthe second principal plane of the aluminum nitride substrate, a mesastructure, an n-electrode, and a p-electrode may be formed on the firstprincipal plane of the aluminum nitride substrate, in one embodiment,instead of forming the electrode on the second principal plane of thealuminum nitride substrate.

(Oxide Film)

The oxide film formed by the manufacturing method of the semiconductorlight-emitting element in the present embodiment is not limited inparticular, as far as it is an oxide film including an amorphous oxidefilm and/or a crystalline oxide film (that is, only the amorphous oxidefilm, only the crystalline oxide film, or both of the amorphous oxidefilm and the crystalline oxide film may be included).

From a viewpoint of improving the light-emitting efficiency, theamorphous oxide film may be an oxide film including Al, in oneembodiment. In addition, from a viewpoint of improving thelight-emitting efficiency, it may be an oxide film having an unevenstructure, in another embodiment. That is, the interface between theoxide film and the aluminum nitride substrate may have an unevenstructure, or the surface of the oxide film (that is, a plane on anopposite side to the plane in contact with the aluminum nitridesubstrate of the oxide film) may have an uneven structure, in yetanother embodiment. In addition, the oxide film may have a layeredstructure having plural oxide films including the amorphous oxide filmand the crystalline oxide film, and the oxide film may have an unevenstructure at the interface between the amorphous oxide film and thecrystalline oxide film. The crystalline oxide film is a polycrystalincluding Al, in one embodiment. Further, the oxide film may have alayered structure having plural oxide films including the amorphousoxide film and the crystalline oxide film, and the oxide film may have astructure having the crystalline oxide film on the amorphous oxide film.

As to such an uneven structure at the interface between the oxide filmand the aluminum nitride substrate, when the height of the unevenstructure to be described later is smaller than 10 nm, the structure isassessed to be flat (there is no uneven structure). When the height isequal to or larger than 10 nm, the structure is assessed to be uneven.From a viewpoint of improving the extraction efficiency, the height ofthe uneven structure may be equal to or larger than 10 nm and equal toor smaller than 2 μm, in one embodiment, may be equal to or larger than50 nm and equal to or smaller than 1 μm, in another embodiment, or maybe equal to or larger than 100 nm and equal to or smaller than 500 nm,in yet another embodiment.

The height of the uneven structure is measured by using an imageobtained by capturing a cross section of the oxide film with a STEM(Scanning Transmission Electron Microscope) (image magnification: 40000times). Firstly, a reference line parallel to an interface between thealuminum nitride substrate and the semiconductor layer is arranged belowthe uneven structure not to overlap the uneven structure. Next, adistance from the reference line to the uneven structure (the interfacebetween the surface of the oxide film and/or the oxide film and thealuminum nitride substrate) is read for 3 μm in width with respect tothe reference line. A ten-point average roughness R is calculated, whichis a difference between an average of the distances (Yp) from the tophaving the longest distance to the top having the fifth longest distanceand an average of the distances (Yv) from the bottom having the shortestdistance to the bottom having the fifth shortest distance. The top andthe bottom mean regions where the inclination is parallel to thereference line. It is to be noted that when there is no top or bottom ina range of 3 μm in width with respect to the reference line, the heightof the uneven structure is supposed to zero. In addition, when the totalnumber of the tops and the bottoms in the range of 3 μm in width withrespect to the reference line is equal to or larger than 1 and equal toor smaller than 20, after moving to an adjacent field of vision from theimage-captured area, the cross-sectional images are captured until thetotal number is at least 20.

The above-described ten-point average roughness R is calculated fordifferent five points of cross sections, so that the average value ofthe ten-point average roughness R at five points of cross sections issupposed to be the height of the uneven structure.

The oxide film may include aluminum as a constitution element. As theoxide film including aluminum, as described above, aluminum oxide,hydration aluminum oxide, aluminum hydroxide, or a film in which theabove aluminum is mixed with aluminum nitride may be used, but the oxidefilm is not limited to them.

The refractive index of such an oxide film is lower than the refractiveindex of aluminum nitride which is a material of the substrate. Inparticular, the refractive indexes of the above-described aluminumoxide, hydration aluminum oxide, and aluminum hydroxide are lower thanthe refractive index of aluminum nitride.

The thickness of the oxide film is not limited in particular. However,as no effect in the light extraction efficiency improvement can beexpected at the thickness of the natural oxide film or so, the thicknessof the oxide film may be equal to or larger than 10 nm or equal to orsmaller than 5 μm, in one embodiment, or may be equal to or larger than100 nm or equal to or smaller than 5 μm, in another embodiment. When theoxide film has a layered structure of the amorphous oxide film and thecrystalline oxide film, from a viewpoint of improving the lightextraction efficiency, the thickness of the amorphous oxide film may beequal to or larger than 10 nm or equal to or smaller than 3 μm, in oneembodiment, may be equal to or larger than 50 nm or equal to or smallerthan 2.5 μm, in another embodiment, or may be equal to or larger than100 nm or equal to or smaller than 2 μm, in yet another embodiment. Thethickness of the crystalline oxide film may be equal to or larger than10 nm or equal to or smaller than 2 μm, in one embodiment, may be equalto or larger than 50 nm or equal to or smaller than 1.5 μm, in anotherembodiment, or may be equal to or larger than 100 nm or equal to orsmaller than 1 μm, in yet another embodiment.

The thickness of the oxide film is measured by capturing across-sectional image of the oxide film with STEM. The measurementdirection (axis) of the thickness is set to a direction perpendicular tothe interface between the aluminum nitride substrate and thesemiconductor layer. For example, as illustrated in FIG. 12A, when theinterface between the oxide film and the aluminum nitride substrate isflat, a value measured at one point is supposed to be the thickness ofthe oxide film. In addition, when the oxide film or the aluminum nitridesubstrate has an uneven structure, since a dent and a projection appearrepeatedly, the depth is different depending on the measurement point.For example, as illustrated in FIG. 12B, when the interface between theoxide film and the aluminum nitride substrate (substrate) has an unevenstructure, such an uneven structure of the substrate is supposed to be areference and an average value of values measured with all dents andprojections of the uneven structure of the substrate is supposed to thethickness of the oxide film. The image magnification is 300000 times,when the thickness of the oxide film is 10 nm to 100 nm, the imagemagnification is 20000 times, when the thickness of the oxide film is100 nm to 3 μm. The image magnification is 5000 times, when thethickness of the oxide film is 3 μm to 5 μm.

The whole aspect of the mechanism of improving the light extractionefficiency of the semiconductor light-emitting element is not stillclear with the oxide film formed in the manufacturing method of thesemiconductor light-emitting element in the present embodiment. However,the structure has the oxide film having the refractive index smallerthan that of the aluminum nitride substrate on the second principalplane of the aluminum nitride substrate. Therefore, it is supposed thatthe critical angle defined in Snell's law can be designed to be large atthe interface between the aluminum nitride substrate and the oxide film,the reflection of the incident light is restrained due to the largecritical angle, and the extraction efficiency increases. Further, it issupposed that since the oxide film formed in the present embodiment hasa density or composition continuously or discontinuously changing, areflection of the light at the interface between the substrate and theoxide film is restrained, and the light extraction efficiency isimproved.

In particular, when the oxide film has the layered structure of theamorphous oxide film and the crystalline oxide film, it is supposed thatthe light extraction efficiency by the above-mentioned mechanism can beremarkably improved. Furthermore, when at least one of the surface ofthe oxide film, the interface of the oxide film (for example, theinterface between the amorphous oxide film and the crystalline oxidefilm), and the interface between the oxide film and the aluminum nitridesubstrate has an uneven structure, the light extraction efficiencyimprovement caused by the light scattering effect can occur, too. Also,by forming the uneven structure on the oxide film in consideration ofthe light scattering effect, the semiconductor light-emitting elementthat further improves the light extraction efficiency is made available.

(Example of Process Flow)

Next, referring to the drawings, a process flow in the presentembodiment will be described with an example. Here, a manufacturingprocess will be sequentially described from an aluminum nitridesubstrate to a completion of a semiconductor light-emitting element inthe present embodiment. In addition, an oxide film forming devicesuitable for use in the present embodiment will be described.

FIG. 1 is a schematic view illustrative of a configuration example of anoxide film forming device 50 suitable for use in the present embodiment.FIG. 2A to FIG. 2D are cross-sectional views illustrative of sequentialprocesses in a manufacturing method of a semiconductor light-emittingelement 100 in the present embodiment.

In this process flow, firstly, the oxide film forming device 50 isprepared beforehand so that an oxide film 20 is formed on a secondprincipal plane 1 b of an aluminum nitride substrate 1 (that is to say,to carry out the setting process and the oxide film forming process).

As illustrated in FIG. 1, the oxide film forming device 50 includes achamber 51 that can be sealed to maintain its inside at an atmosphericpressure or higher, a stage 53 arranged at the inside of the chamber 51to be capable of supporting the aluminum nitride substrate (for example,wafer) 1, a nozzle 55 arranged at an upper center in the chamber 51, anH₂O supply source 61 configured to supply water (H₂O) molecules into thechamber 51 through the nozzle 55, a heater 81 arranged at acircumference of the chamber 51 and configured to heat the inside of thechamber 51, and a controller 90 configured to control the H₂O supplysource 61 and the heater 81, respectively, so that the relative humidityand the temperature in the chamber 51 fall within ranges set beforehand(that is, predefined ranges). Also, although not illustrated, a heatermay be built in the stage 53, so that such a heater built in the stage53 may heat the inside of the chamber.

Here, the value of the relative pressure (gauge pressure) in the chamber51 is determined by the relative humidity and the temperature in thechamber 51. The relative pressure in the chamber 51 is not anindependent parameter. The relative pressure in the chamber 51 iscontrolled by the controller 90 (or is set beforehand by a devicemanager who manages the oxide film forming device 50). By setting thetemperature and the relative humidity in the chamber 51 to be higherthan a stand-by state, the relative pressure can be relatively higherthan the atmospheric pressure. Further, although not illustrated, theoxide film forming device 50 may be provided with a pressure pumpcapable of intentionally controlling the relative pressure in thechamber 51.

Further, the H₂O supply source 61 may include a water tank, which is notillustrated in the oxide film forming device 50, and a heater for thewater tank (a different heater from the heater for heating the chamber),so that the water in the water tank may be heated with the heater forthe water tank to supply the water which has changed into gas into thechamber 51 through the nozzle 55. In this case, the relative humidity inthe chamber 51 depends on the output from the heater for the water tankand an ambient temperature in the chamber. The output from the heaterfor the water tank may be controlled by the controller 90, or may take avalue set beforehand by the device manager.

The aluminum nitride substrate 1 is prepared next. As illustrated inFIG. 2A, the aluminum nitride substrate 1 has a first principal plane 1a, and a second principal plane 1 b located on the opposite side to thefirst principal plane 1 a. As illustrated in FIG. 2B, an n-typesemiconductor layer 11, a light-emitting layer 13, an electron blockinglayer 15, and a p-type semiconductor layer 17 are sequentially stackedon the first principal plane 1 a of the aluminum nitride substrate 1 toform the semiconductor layer 10 including these layers. Thesemiconductor layer 10 is formed in MBE method or MOCVD method, forexample.

Next, as illustrated in FIG. 2C, the semiconductor layer 10 is patternedin a mesa shape by using a photolithography technique and an etchingtechnique. Then, an insulating film 31 is deposited on the firstprincipal plane 1 a of the aluminum nitride substrate 1 to cover thesemiconductor layer 10 that has been patterned in the mesa shape (thatis to say, in a mesa structure). The insulating film 31 is a siliconoxide (SiO2) film, for example, and is formed in CVD method, forexample.

Then, by using well-known photolithography technique and etchingtechnique, the insulating film 31 is partially removed to form contactholes having bottoms of the n-type semiconductor layer 11 and the p-typesemiconductor layer 17, respectively.

Subsequently, by using a photolithography technique and a lift offtechnique, metal membranes are selectively deposited to embed thecontact holes. The metal membranes are deposited in a vacuum depositionmethod, for example. Thus, an electrode portion 33 electricallyconnected to the n-type semiconductor layer 11 and an electrode portion35 electrically connected to the p-type semiconductor layer 17 areformed.

Next, the aluminum nitride substrate 1 in which the electrode portion 35is formed is set on the stage 53 of the oxide film forming device 50illustrated in FIG. 1. Here, as illustrated in FIG. 1, the aluminumnitride substrate 1 is set on the stage 53 with the second principalplane 1 b of the aluminum nitride substrate 1 facing upward (that is tosay, facing the nozzle 55 side) (the setting process).

Then, the chamber 51 is heated with the water molecules being introducedin the chamber 51. Accordingly, as illustrated in FIG. 2D, the secondprincipal plane 1 b of the aluminum nitride substrate 1 is subject to aheating process to form the oxide film 20 including an amorphous oxidefilm 21 on the second principal plane 1 b (the oxide film formingprocess).

Here, the controller 90 illustrated in FIG. 1 is configured to controlthe H₂O supply source 61 and the heater 81, so that the heating processconditions (the relative humidity, the temperature, the relativepressure, the process time) of the aluminum nitride substrate 1 fallwithin predefined ranges. Also, the controller 90 may be configured tocontrol the heating process conditions to further form a crystallineoxide film 23 having a surface of an uneven structure on the amorphousoxide film 21 of the second principal plane 1 b. The crystalline oxidefilm 23 is formed at the same time with the amorphous oxide film 21 inthe oxide film forming process.

Then, after the oxide film 20 is formed on the second principal plane 1b of the aluminum nitride substrate 1, the aluminum nitride substrate 1is taken out from the chamber 51 of the oxide film forming device 50.Through the above-mentioned processes, the semiconductor light-emittingelement 100 in the present embodiment is manufactured.

(Effects of Embodiments)

According to embodiments of the present invention, the inside of thechamber 51 is heated with the water molecules being introduced in thechamber 51 in which the aluminum nitride substrate 1 is arranged. Thus,it is possible to form the oxide film 20 including the amorphous oxidefilm 21 and/or the crystalline oxide film 23, which are smaller inrefraction index than the aluminum nitride substrate 1, on the secondprincipal plane 1 b of the aluminum nitride substrate 1. As a result, itis made possible to remarkably improve the light extraction efficiencyfrom the second principal plane 1 b of the aluminum nitride substrate 1.

In addition, in the process of forming the oxide film 20 as describedabove (that is to say, the oxide film forming process), the etchingprocess does not have to be carried out on the surface of the oxide film20 or the second principal plane 1 b of the aluminum nitride substrate1. Accordingly, the mass productivity is good and an etching damage tothe aluminum nitride substrate 1 or the semiconductor layer 10 can besuppressed.

Furthermore, the above-described oxide film forming process is carriedout after the semiconductor layer 10 is formed. In this manner, theoxide film is formed on the second principal plane, instead that anoptical pattern of an uneven structure with the light extraction effectis arranged on the surface on which the semiconductor layer of thesubstrate is made to grow (the interface between the substrate and thesemiconductor layer). Thus, since the oxide film forming process doesnot affect a crystal defect occurrence at the time of the semiconductorlayer growth, a degradation in the crystal property of the semiconductorlayer 10 can be suppressed.

EXAMPLES

The present invention will be described in more detail based onExamples. It is to be noted that the present invention is not limited tothe following Examples, and can be changed as necessary.

Example 1

By use of a MOCVD (Metal Organic Chemical Vapor Deposition) device, awafer in which an n-type semiconductor layer including aluminum,gallium, and nitrogen, an MQW (Multiple Quantum Well) light-emittinglayer, an electron blocking layer, and a p-type semiconductor layer aresequentially formed on an aluminum nitride substrate, and was subject towell-known lithography technique and dry etching technique to make amesa structure from which the n-type semiconductor layer is exposed, sothat electrodes were vapor-deposited on both p-type semiconductor layerand the n-type semiconductor layer, and the second principal plane ofthe aluminum nitride substrate was ground. In this manner, sixsemiconductor light-emitting elements of the ultraviolet region weremanufactured.

Then, an electrical current of 100 mA was applied to each of thesemiconductor light-emitting elements, and the light-emitting intensityof each semiconductor light-emitting element was measured and recordedas an initial value.

Next, each semiconductor light-emitting element was set in a chamber andheld for 1000 hours under conditions that the temperature was 121° C.,the relative humidity was 100%, and the relative pressure was 0.1 MPa(the setting process, the oxide film forming process).

In the meanwhile, after 50 hours, 100 hours, 250 hours, 350 hours, 450hours, 550 hours, 750 hours, and 1000 hours elapsed from the start ofthe process, each semiconductor light-emitting element was taken outonce. The electrical current of 100 mA was applied, and thelight-emitting intensity of each semiconductor light-emitting elementwas measured and recorded.

FIG. 3 and FIG. 4 illustrate SEM (Scanning Electron Microscope) imageson the second principal plane of the aluminum nitride substrate of thesemiconductor light-emitting element, after the semiconductorlight-emitting element is subject to the process for 500 hours and thentaken out from the chamber. FIG. 3 and FIG. 4 exhibit that the filmhaving an uneven structure is formed on the second principal plane.

FIG. 5 illustrates a cross-sectional STEM image (20000 times) of thealuminum nitride substrate. From FIG. 5, it is understood that a firstlayer having a thickness of 550 nm on the second principal plane of thealuminum nitride substrate, and a second layer having a thickness of 300nm and an uneven surface are formed. When the height of the unevenstructure was measured from the cross-sectional STEM images (40000times) of five places, the height of the uneven structure on the surfaceof the second layer was 160 nm and the height of the uneven structure atthe interface between the first layer and the second principal plane ofthe aluminum nitride substrate was 140 nm. In addition, the height ofthe uneven structure at the interface between the first layer and thesecond layer was less than 10 nm, and it was flat. When the compositionsand properties of the respective layers were analyzed by EDX (EnergyDispersive X-ray spectrometry) and electron beam diffraction, it wasunderstood that the first layer was an amorphous oxide film withAl:O=1:3, and the second layer was a crystalline oxide film withAl:O=1:3.

In addition, FIG. 6 illustrates a graph of outputs changing over timewith respect to the process time with initial values being 0 hours. Thehorizontal axis of FIG. 6 indicates the process time (hr), whereas thevertical axis indicates the change rate (%) in the optical output. (1)to (6) in FIG. 6 are respective data of the six semiconductorlight-emitting elements, as described above. It is understood that theoutputs after the process (50 hours to 1000 hours) are increased by 30to 80% from the initial values before the process (0 hours). That is tosay, it is understood that in a state where there are water molecules onthe second principal plane of the aluminum nitride substrate, the lightextraction efficiency is remarkably improved by processing thesemiconductor light-emitting elements in the chamber.

Example 2

Except that the second principal plane of the semiconductorlight-emitting element was ground and then CMP was further carried out,the semiconductor light-emitting element obtained by a method similar toExample 1 was held for 50 hours under conditions that the temperaturewas 121° C., the relative humidity was 100%, and the relative pressurewas 0.1 MPa.

FIG. 7 illustrates a SEM image of the second principal plane of thealuminum nitride substrate of the semiconductor light-emitting elementafter the process. From FIG. 7, it is understood that an oxide filmhaving a surface of an uneven structure is formed in a similar manner toExample 1.

FIG. 8 illustrates a cross-sectional STEM image (20000 times) of thealuminum nitride substrate. From FIG. 8, it is understood that anamorphous oxide film (the first layer) having a thickness of 1400 nm onthe second principal plane of the aluminum nitride substrate, and acrystalline oxide film (the second layer) having a thickness of 250 nmof an uneven surface are formed. The heights of the uneven structureswere measured from the cross-sectional STEM images (40000 times) of fiveplaces. The height of the uneven structure on the surface of the secondlayer was 100 nm, and the heights of the uneven structures at theinterface between the first layer and the aluminum nitride substrate andat the interface between the second layer and the first layer were lessthan 10 nm, and it was flat.

In addition, when the light-emitting intensities were compared beforeand after the process, the light-emitting intensity was improved by 10%after the process. When Example 2 is compared with Example 1, it isunderstood that the interface between the aluminum nitride substrate andthe first layer having an uneven structure like Example 1 is suitablefrom a viewpoint of improving the light-emitting efficiency, in oneembodiment. In addition, in order to form the uneven structure at theinterface between the aluminum nitride substrate and the first layerlike Example 1, it is understood that the state of the second principalplane of the aluminum nitride substrate before the inside of the chamberis subject to a heating process with the water molecules beingintroduced in the chamber would affect forming of the uneven structure.To be specific, it is understood that the uneven structure tends to beeasily formed at the interface between the aluminum nitride substrateand the first layer after the second principal plane is ground.

Example 3

Except that the second principal plane of the semiconductorlight-emitting element was ground and then CMP was further carried out,the semiconductor light-emitting element obtained by a method similar toExample 1 was held for 50 hours under conditions that the temperaturewas 121° C., the relative humidity was 65%, and the relative pressurewas 0.03 MPa.

FIG. 9 illustrates a SEM image of the second principal plane of thealuminum nitride substrate of the semiconductor light-emitting elementafter the process. From FIG. 9, it is understood that an oxide filmhaving a surface of an uneven structure is formed in Example 3 in asimilar manner to Example 1.

When the light-emitting intensities were compared before and after theprocess, the light-emitting intensity was improved by 15% after theprocess. Accordingly, it is understood that the relative humidity of 65%or more is necessary for forming the oxide film capable of improving thelight extraction efficiency.

Example 4

Except that the second principal plane of the semiconductorlight-emitting element was ground and then CMP was further carried out,the semiconductor light-emitting element obtained by a method similar toExample 1 was held for 50 hours under conditions that the temperaturewas 105° C., the relative humidity was 100%, and the relative pressurewas 0.02 MPa.

FIG. 10 illustrates a SEM image of the second principal plane of thealuminum nitride substrate of the semiconductor light-emitting elementafter the process. In Example 4, the surface state of the secondprincipal plane before the process was same as those in Examples 2 and3, whereas the surface state after the process was largely differentfrom those in Examples 2 and 3 and was flat similar to the secondprincipal plane before the process. Accordingly, it is supposed that thesurface shape largely depends on the temperature.

FIG. 11 illustrates a cross-sectional STEM image (500000 times) of thealuminum nitride substrate. From FIG. 11, it is understood that thefirst layer having a thickness of 32.7 nm is formed on the secondprincipal plane of the aluminum nitride substrate. In addition, it isunderstood that the interface between the first layer and the aluminumnitride substrate has a flat structure.

Further, as for the sample of Example 4, the light-emitting intensitywas improved by 15% by the above process. Accordingly, it is supposedthat when the oxide film of at least 32.7 nm is formed, the lightextraction efficiency improves.

Comparative Example 1

Except that the second principal plane of the semiconductorlight-emitting element was ground and then CMP was further carried out,the semiconductor light-emitting element obtained by a method similar toExample 1 was held for 50 hours under conditions that the temperaturewas 25° C., the relative humidity was 100%, and the relative pressurewas 0 MPa.

The oxide film of 10 nm or more was not formed by the above process, andthe light-emitting intensity did not improve. That is, it is understoodthat unless the inside of the chamber is heated, any oxide film thickenough to be capable of improving the light extraction efficiency is notformed on the second principal plane of the aluminum nitride substrate.

Comparative Example 2

Except that the second principal plane of the semiconductorlight-emitting element was ground and then CMP was further carried out,the semiconductor light-emitting element obtained by a method similar toExample 1 was held for 50 hours under conditions that the temperaturewas 121° C., the relative humidity was 0%, and the relative pressure was0 MPa.

The oxide film of 10 nm or more was not formed by the above process, andthe light-emitting intensity did not improve. That is to say, it isunderstood that in a state where the relative humidity is too low (thewater molecules are not introduced substantially), any oxide film thickenough to be capable of improving the light extraction efficiency is notformed on the second principal plane of the aluminum nitride substrate.

Table 1 illustrates process conditions and the light-emitting intensityimprovement rates of Examples and Comparative examples together.

(Table 1)

TABLE 1 light-emit- process conditions ting intensity second temper-relative improvement main ature humidity time rate plane [° C.] [%][hrs] [%] example 1 grinding 121 100 50~1000 30~80 example 2 CMP 121 10050 10 example 3 CMP 121 65 50 15 example 4 CMP 105 100 50 15 comparativeCMP 25 100 50 not improved example 1 comparative CMP 121 0 50 notimproved example 2

(Others)

It is to be noted that the present invention is not limited to theabove-described embodiments. It should be apparent that modificationsand adaptations to those embodiments may occur based on knowledge of oneskilled in the art and such modifications and adaptations should beincluded in the scope of the present invention.

In one embodiment of the present invention, there is provided amanufacturing method of a semiconductor light-emitting element, themanufacturing method including: setting in a chamber an aluminum nitridesubstrate in which a semiconductor layer is formed on a first principalplane; and heating an inside of the chamber with a water molecule beingintroduced in the chamber to form an oxide film including an amorphousoxide film and/or a crystalline oxide film on a second principal planelocated on an opposite side to the first principal plane of the aluminumnitride substrate.

In addition, in the above-described manufacturing method of thesemiconductor light-emitting element, in forming the oxide film, theoxide film having a surface of an uneven structure may be formed.

Further, in the above-described manufacturing method of thesemiconductor light-emitting element, in forming the oxide film,relative humidity in the chamber may be equal to or higher than 50% andequal to or lower than 100%.

Furthermore, in the above-described manufacturing method of thesemiconductor light-emitting element, in forming the oxide film, atemperature in the chamber may be equal to or higher than 100° C. andequal to or lower than 140° C.

Moreover, in the above-described manufacturing method of thesemiconductor light-emitting element, in forming the oxide film,relative pressure in the chamber may be equal to or higher than 0.01 MPaand equal to or lower than 0.3 MPa.

In another embodiment of the present invention, there is provided asemiconductor light-emitting element, including: a semiconductor layerformed on a first principal plane of an aluminum nitride substrate; andan oxide film formed on a second principal plane located on an oppositeside to the first principal plane of the aluminum nitride substrate, theoxide film being smaller in refractive index than the aluminum nitridesubstrate, wherein the oxide film includes an amorphous oxide filmand/or a crystalline oxide film.

In addition, in the above-described semiconductor light-emittingelement, an interface between the oxide film and the aluminum nitridesubstrate may have an uneven structure.

Further, in the above-described semiconductor light-emitting element, asurface of the oxide film may have an uneven structure.

Furthermore, in the above-described semiconductor light-emittingelement, the oxide film may be a layered structure made of a pluralityof the oxide films including the amorphous oxide film and thecrystalline oxide film, and the oxide film may have an uneven structureat an interface between the amorphous oxide film and the crystallineoxide film.

In addition, in the above-described semiconductor light-emittingelement, the oxide film may be a layered structure made of a pluralityof the oxide films including the amorphous oxide film and thecrystalline oxide film, and the oxide film may include the crystallineoxide film on the amorphous oxide film.

Further, in the above-described semiconductor light-emitting element,the oxide film may include Al.

Furthermore, in the above-described semiconductor light-emittingelement, a thickness of the oxide film may be equal to or larger than 10nm and equal to or smaller than 5 μm.

Moreover, in the above-described semiconductor light-emitting element,the oxide film may include at least the amorphous oxide film, and athickness of the amorphous oxide film may be equal to or larger than 10nm and equal to or smaller than 3 μm.

In addition, in the above-described semiconductor light-emittingelement, the oxide film may include at least the crystalline oxide film,and a thickness of the crystalline oxide film may be equal to or largerthan 10 nm and equal to or smaller than 2 μm.

Further, in the above-described semiconductor light-emitting element,the semiconductor layer may be a group III-V compound semiconductorlayer including an element selected from the group consisting of atleast aluminum, gallium, nitrogen, and indium.

Furthermore, in the above-described semiconductor light-emittingelement, the second principal plane of the aluminum nitride substratemay be a C plane and an N plane in a hexagonal crystal.

In yet another embodiment of the present invention, there is provided, asemiconductor light-emitting element obtained by carrying out a methodincluding: a setting process configured to set in a chamber an aluminumnitride substrate in which a semiconductor layer is formed on a firstprincipal plane, and an oxide film forming process configured to heat aninside of the chamber with a water molecule being introduced in thechamber and to form an oxide film including an amorphous oxide filmand/or a crystalline oxide film on a second principal plane located onan opposite side to the first principal plane of the aluminum nitridesubstrate.

Advantageous Effects

In one embodiment of the present invention, the inside of the chamber isheated with the water molecules being introduced in the chamber wherethe aluminum nitride film is arranged. Thus, it is possible to form anoxide film, including an amorphous oxide film and/or a crystalline oxidefilm and having a refractive index smaller than that of the aluminumnitride film, on the second principal plane of the aluminum nitridefilm. As a result, it is possible to remarkably improve the lightextraction efficiency from the second principal plane of the aluminumnitride film.

In addition, in the process of forming the above-described oxide film(that is, oxide film forming process), no mask has to be formed with theuse of a photolithography technique. Further, no dry etching process hasto be carried out on the surface of the oxide film or the secondprincipal plane of the aluminum nitride film. Accordingly, the massproductivity is good, and an etching damage onto the aluminum nitridefilm or the semiconductor layer can be suppressed.

Furthermore, in the oxide film forming process, after the semiconductorlayer is formed, an oxide film is formed on the second principal plane.An uneven structure is formed on the second principal plane, insteadthat an optical pattern with the light extraction efficiency is arrangedon a surface (at the interface between the substrate and thesemiconductor layer) of the substrate on which the semiconductor layeris made to grow. Hence, a degradation in crystal property of thesemiconductor layer can be suppressed without affecting a crystal defectoccurrence at the time of the semiconductor layer growth in the oxidefilm forming process.

INDUSTRIAL APPLICABILITY

The present invention relates to a manufacturing method of asemiconductor light-emitting element, and the semiconductorlight-emitting element, in particular, relates to an element with a highlight-emitting efficiency in a nitride semiconductor light-emittingelement formed on an aluminum nitride substrate.

REFERENCE SIGNS LIST

-   1 aluminum nitride substrate-   1 a first principal plane-   1 b second principal plane-   10 semiconductor layer-   11 n-type semiconductor layer-   13 light-emitting layer-   15 electron blocking layer-   17 p-type semiconductor layer-   20 oxide film-   21 amorphous oxide film-   23 crystalline oxide film-   31 insulating film-   33, 35 electrode portion-   50 oxide film forming device-   51 chamber-   53 stage-   55 nozzle-   61 H₂O supply source-   81 heater-   90 controller-   100 semiconductor light-emitting element

1. A manufacturing method of a semiconductor light-emitting element, themanufacturing method comprising: setting in a chamber an aluminumnitride substrate in which a semiconductor layer is formed on a firstprincipal plane; and heating an inside of the chamber with a watermolecule being introduced in the chamber to form an oxide film includingan amorphous oxide film and/or a crystalline oxide film on a secondprincipal plane located on an opposite side to the first principal planeof the aluminum nitride substrate.
 2. The manufacturing method of thesemiconductor light-emitting element according to claim 1, wherein informing the oxide film, the oxide film having a surface of an unevenstructure is formed.
 3. The manufacturing method of the semiconductorlight-emitting element according to claim 1, wherein in forming theoxide film, relative humidity in the chamber is equal to or higher than50% and equal to or lower than 100%.
 4. The manufacturing method of thesemiconductor light-emitting element according to claim 1, wherein informing the oxide film, a temperature in the chamber is equal to orhigher than 100° C. and equal to or lower than 140° C.
 5. Themanufacturing method of the semiconductor light-emitting elementaccording to claim 1, wherein in forming the oxide film, relativepressure in the chamber is equal to or higher than 0.01 MPa and equal toor lower than 0.3 MPa.
 6. A semiconductor light-emitting element,comprising: a semiconductor layer formed on a first principal plane ofan aluminum nitride substrate; and an oxide film formed on a secondprincipal plane located on an opposite side to the first principal planeof the aluminum nitride substrate, the oxide film being smaller inrefractive index than the aluminum nitride substrate, wherein the oxidefilm includes an amorphous oxide film and/or a crystalline oxide film.7. The semiconductor light-emitting element according to claim 6,wherein an interface between the oxide film and the aluminum nitridesubstrate has an uneven structure.
 8. The semiconductor light-emittingelement according to claim 6, wherein a surface of the oxide film has anuneven structure.
 9. The semiconductor light-emitting element accordingto claim 6, wherein the oxide film is a layered structure made of aplurality of the oxide films including the amorphous oxide film and thecrystalline oxide film, and wherein the oxide film has an unevenstructure at an interface between the amorphous oxide film and thecrystalline oxide film.
 10. The semiconductor light-emitting elementaccording to claim 6, wherein the oxide film is a layered structure madeof a plurality of the oxide films including the amorphous oxide film andthe crystalline oxide film, and wherein the oxide film includes thecrystalline oxide film on the amorphous oxide film.
 11. Thesemiconductor light-emitting element according to claim 6, wherein theoxide film includes Al.
 12. The semiconductor light-emitting elementaccording to claim 6, wherein a thickness of the oxide film is equal toor larger than 10 nm and equal to or smaller than 5 μm.
 13. Thesemiconductor light-emitting element according to claim 6, wherein theoxide film includes at least the amorphous oxide film, and a thicknessof the amorphous oxide film is equal to or larger than 10 nm and equalto or smaller than 3 μm.
 14. The semiconductor light-emitting elementaccording to claim 6, wherein the oxide film includes at least thecrystalline oxide film, and a thickness of the crystalline oxide film isequal to or larger than 10 nm and equal to or smaller than 2 μm.
 15. Thesemiconductor light-emitting element according to claim 6, wherein thesemiconductor layer is a group III-V compound semiconductor layerincluding an element selected from the group consisting of at leastaluminum, gallium, nitrogen, and indium.
 16. The semiconductorlight-emitting element according to claim 6, wherein the secondprincipal plane of the aluminum nitride substrate is a C plane and an Nplane in a hexagonal crystal.
 17. A semiconductor light-emitting elementobtained by carrying out a method comprising: a setting processconfigured to set in a chamber an aluminum nitride substrate in which asemiconductor layer is formed on a first principal plane, and an oxidefilm forming process configured to heat an inside of the chamber with awater molecule being introduced in the chamber and to form an oxide filmincluding an amorphous oxide film and/or a crystalline oxide film on asecond principal plane located on an opposite side to the firstprincipal plane of the aluminum nitride substrate.